Positioning system

ABSTRACT

An ultrawide band two-way ranging based positioning system includes a number of active tags each having a position, and a number of beacons configured for location of a position of a tag of the plurality of active tags. The active tags and the beacons are synchronized continuously to a common time base.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based on and claims the benefit of U.S.provisional patent application Ser. No. 62/335,199, filed May 12, 2016;the present application is also a Continuation-in-Part of U.S. Ser. No.14/493,730, filed Sep. 23, 2014, which claims priority to U.S.provisional patent application Ser. No. 61/946,987, filed Mar. 3, 2014,the contents of which are hereby incorporated by reference in theirentirety.

BACKGROUND

The present disclosure relates to positioning systems. Morespecifically, the present disclosure relates to systems used to identifythe locations of or track objects in a given area.

A Positioning System (PS) is a network of devices used to wirelesslylocate objects or people inside a building or within dense industrialareas. A special design is required since global positioning system(GPS) systems are typically not suitable to establish indoor locationsor other crowded locations as they require an unobstructed line of sightto four or more GPS satellites. Microwaves will be attenuated andscattered by roofs, walls and other objects and multiple reflections atsurfaces cause multipath propagation serving for uncontrollable errors.

Time of flight (ToF) is the amount of time a signal takes to propagatefrom transmitter to receiver. Because the signal propagation rate isconstant and known, the travel time of a signal can be used directly tocalculate distance. Multiple (in GPS at least four satellites)measurements vs. multiple anchor stations can be combined withtrilateration to find a location.

As speed of light is 3×10⁸ m/sec, in radio frequency (RF) based systems,inaccuracy in clock synchronization is a key factor of the positioningerror. In GPS, ToF generally requires a complicated synchronizationmechanism to maintain a reliable source of time for sensors.

In addition, the accuracy of the ToF based methods often suffer fromlarge multipath conditions in localization situations with densepopulations, such as indoor locations and industrial environments whichcan be crowded, which is caused by the reflection and diffraction of theRF signal from objects.

Due to the attenuation and reflections caused by construction materials,it is desirable to have an unobstructed line of sight to at least threeanchor points at any location that should be covered by the system. As aresult, a larger number of anchor stations are required.

SUMMARY

An ultrawide band two-way ranging based positioning system according toone embodiment includes a plurality of active tags each having aposition, and a plurality of beacons configured for location of aposition of a tag of the plurality of active tags. The plurality ofactive tags and the plurality of beacons are synchronized continuouslyto a common time base.

A method of determining a position of an active tag in a system havingan application server and a plurality of fixed location beaconsincludes, in one embodiment, providing a common time base for theplurality of fixed location beacons, and providing a synchronizationmessage from each of the plurality of fixed location beacons. The activetag is registered to the common time base based on the synchronizationmessage. The active tag is ranged to at least a portion of the pluralityof beacons. The active tag reports its position to at least a beacon ofthe plurality of beacons.

A positioning system according to another embodiment comprises aplurality of active tags and a plurality of fixed position beacons incommunication with the plurality of active tags over an ultrawide bandtwo-way ranging based network. Each active tag of the plurality ofactive tags in the system has a globally designated window for two-wayranging polls between it and at least one of the plurality of beacons.The plurality of beacons accepts two-way ranging polls within theglobally designated window. The plurality of active tags reportpositions within a global reporting time slot.

This Summary and the Abstract are provided to introduce a selection ofconcepts in a simplified form that are further described below in theDetailed Description. The Summary and the Abstract are not intended toidentify key features or essential features of the claimed subjectmatter, nor are they intended to be used as an aid in determining thescope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a positioning system in accordance withone embodiment of the present disclosure.

FIG. 2 is a block diagram of an anchor station in accordance with oneembodiment of the present disclosure.

FIG. 3 is a block diagram of a mobile station in accordance with oneembodiment of the present disclosure.

FIG. 4 is a block diagram showing a central location, anchor stationsand a mobile station.

FIG. 5 is a timing diagram showing timing between beacon communicationsignals and tag positioning reports.

FIG. 6 is a diagram showing two beacons listening on a tag global timeslot window.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In one specific configuration, the system is based on a commerciallyproved industrial mesh network such as WirelessHART® network (IEC62591), and is shown in block diagram form in FIG. 1.

FIG. 1 shows a system 300 having plurality of anchor stations (or“beacons”) 302 (three anchor stations 302A, 302B, and 302C shown in FIG.3, and described in greater detail below) which are disposed in knownpositions, and a mobile station (also called a “tag”, “portable unit” orPU”) 304 (described in greater detail below) for which a position 310 isdesired to be determined. Anchor stations 302 are in one embodiment partof a wireless network mesh 306 such as a WirelessHART® network. Inoperation, the plurality of anchor stations 302 are positioned at knownlocations 308A, 308B, and 308C around an area to be monitored. A mobilestation 304 is located on or in close proximity to an object for whichthe position 310 is desired to be known. The object may be, by way ofexample only and not by way of limitation, a piece of inventory or aperson.

In one embodiment as shown in FIG. 1, to determine a distance from ananchor station 302 to the mobile station 304, the anchor station 302transmits an RF message 320 (i.e., a WirelessHART® message). Asdiscussed below, WirelessHART® signals allow for synchronization as theymay contain synchronization information, and allow for transmission ofdata as well as is known in the art. Using this synchronization and datatransmission capability allows the RF pulse to be used as a uniquesynchronization pulse, so that it is possible to know which anchorstation 302 or mobile station 304 is transmitting in addition to whenthe transmission is made. The time difference between transmission ofthe RF pulse and receipt of a response allows for a determination of thedistance between the mobile station and the transmitting anchor station302. A location for the mobile station may be determined using distancecalculations from three separate anchor stations, for example usingtrilateration.

TDMA is a known standard for sharing a frequency channel on a network,by dividing the signal into multiple time slots. Each station, such asanchor stations 302 described herein, may share the frequency being usedfor pulses by being assigned a specific time slot for transmissions. Inone embodiment, a central station 312, having received information fromone or more mobile stations 304 regarding its/their position(s), usesthe determined positions to adjust time slotting within TDMA. Thecentral station 312 is used in the system 300, and is connected with themesh 306 to receive information from one or more mobile stations 304.Time slots for anchor stations 302 are assigned by the central station312. Any appropriate communication technique may be employed includingCDMA techniques or other time and/or frequency slotting or sharingtechniques. Alternatively or in addition, the anchor stations 302 maycommunicate directly with the central station 312.

In one embodiment, location information for one or more mobile stations304 is used to adjust time slots for the various anchor stations. Forexample, when a position 310 of a mobile station is closer to someanchor stations 302 than others, the time slots for those anchorstations 302 at a greater distance may be expanded, allowing for pulsepropagation and receipt without overlap from other anchor stations 302due to distance. The central station 312 in one embodiment monitorsdistances from each line of sight anchor station 302 to each mobilestation 304, and adjusts the time slots for the anchor stationsaccording to expected propagation times, to reduce crosstalk betweenanchor stations 302. Further, pulse coding and different frequenciesincluding spread spectrum techniques may also be used to reducecrosstalk between close anchor stations 302 or other anchor stations.

The anchor system stations 302 are all registered devices on the network300, so each anchor point 302 is a WirelessHART® device with an RFtransmitter 402. In addition, the anchor station 302 contains anoptional communication module 404 that may communicate usingcommunication signals 330, and some glue logic 406.

Details of an anchor station 302 are shown in FIG. 2. Each anchorstation 302 includes in one embodiment an RF transmitter (i.e.,WirelessHART®) 402, communication module 404, glue logic 406 to allowinterface between various more complex logic circuits such as the RFtransmitter 402 and communication module 404, a processing unit 408, orthe like within the anchor station 302, and a clock 410. Glue logic 406is known in the art and will not be described further herein. Also,WirelessHART® is a known standard for wireless sensor networking basedon the Highway Addressable Remote Transducer (HART®) Protocol. In oneembodiment, the anchor stations 302 are part of a mesh network ofdevices, such that each anchor station 302 is a WirelessHART® device onthe wireless mesh network 306. Clock 410 is operatively coupled in oneembodiment to processing unit 406 to allow processing unit 406 todetermine a time of transmission of signals such as RF signal 320. Inanother embodiment, clock 410 may be a part of processing unit 406.

As illustrated in FIG. 3, in one configuration, the mobile station 304is also a WirelessHART® device with an optional communication unit 502,optional GPS unit 504, a small processing unit 506, and an RF receiver508.

The mobile station 304 includes in one embodiment an optionalcommunication module 502, an optional GPS unit 504 for use in outdoorlocations, a processing unit 506, an RF receiver 508, and a clock 510.RF receiver 508 is operatively coupled to processing unit 506. Clock 510is operatively coupled in one embodiment to processing unit 506 to allowprocessing unit 506 to determine a time of receipt of signals such as RFsignal 320. In another embodiment, clock 510 may be a part of processingunit 506.

In order to calculate its positioning, the mobile station 304 measuresthe distance to at least three anchor stations 302. Three anchorstations 302 allow for adequate determination of position, for exampleby trilateration, for a mobile station 304 that is in the line of sightof each of the anchor stations 302, but in case there are more anchorpoints 302 in the area, the redundancy can be used to improve therobustness of the measurement. While three anchor stations 302(respectively, 302A, 302B, and 302C) are shown in FIG. 3, it should beunderstood that for complete coverage of an area, additional anchorstations may be used to increase the accuracy of position 310determination, and to provide more potential line of sight options forall parts of the area to be covered by the system 300. Further, whileone mobile station 304 is shown, multiple mobile stations 304 may bedisposed within the area to be monitored, and the system 300 may use thesame plurality of anchor stations 302 transmitting RF signals todetermine multiple locations 310 of multiple mobile stations 304. Themobile station 304 reports the acquired information to a central station312 via the network 306. The central station 312 may run an algorithmthat optimizes the TDMA time slots according the current positions 310of mobile stations 304 and may modify the network parametersaccordingly. Within the mesh network, since locations of anchor stations302 may be transmitted over the network, anchor stations 302, mobilestations 304 or the central station 312 may, in various embodiments,make a determination of the position of a mobile station 304.

In some situations, position of a mobile station may be determined usingfewer than three anchor stations. Such a situation includes by way ofexample, but is not limited to, where there is some external informationknown about the mobile station, such as that it is located in a corridoror other known confined area, or where other tracking information isknown for the mobile station. In such situations, position may bedetermined using as few as one anchor station.

A positioning system (PS) is provided in one embodiment in which anetwork of devices is used to wirelessly locate objects or people insidea building or within dense industrial areas. A system embodiment isprovided that does not necessarily rely on GPS for locationdetermination. Further, microwaves will be attenuated and scattered byroofs, walls and other objects especially in an indoor environment.Still further, multiple reflections at surfaces can cause multi-pathpropagation resulting in uncontrollable errors.

An embodiment 600 of a PS using ultra-wide band communication betweenbeacons and tags of a system is shown in block diagram in FIG. 4. PS 600comprises in one embodiment an application server 602, a plurality ofbeacons 604, and at least one mobile tag 606. In this embodiment, a tag606 is a portable component that can be attached to or otherwise carriedby persons or equipment that is to be tracked by the system 600. Abeacon 604 is a fixed position, static anchor placed at a predefinedcoordinate of a positioning arena or area 620. In one embodiment, aplurality of beacons 604, arranged in an array such as that shown inFIG. 4, are positioned in the arena 620 to allow for all locationswithin the arena 620 to be visible by more than one beacon 604, andpreferably by at least three beacons 604. Under some conditions, rangemeasurements from the tag to at least three beacons 604 will provideenough information to determine a position of the tag 606 within thearena 620.

The application server 602 in one embodiment prepares and sends setupand configuration information to the beacons 604. The application server602 also in one embodiment receives and processes position informationfor the tag 606, and other information from the beacons 604, for thepurpose of displaying the position of the tag 606 to an end user, forarchiving, or for any other further analysis. While one tag 606 isshown, it should be understood that many tags 606 may be at positionswithin the arena 620. The application server 602, in conjunction withthe beacons 604, is used in one embodiment to monitor the positions of aplurality of tags 606 within the arena 620. The application server 602in one embodiment communicates with the beacons 604 via a low powernetwork as indicated in lines 608, described further below.

FIG. 4 is a diagram showing communication between an application server602, beacons 604, and a portable tag 606. In one embodiment, a wirelessnetwork such as a WirelessHART® network is used as a backbone for thepositioning system that uses UWB (e.g., IEEE 802.15.4a-2011) for rangemeasurements. The low power network used to communicate with beacons 604is illustrated at 608, along with the ultra-wide band ranging signal 612and the data link 610 between a tag 606 and one or more beacons 604. Inthe example illustrated, beacons 604 are distributed across the facility(e.g., the arena 620) with a spacing of approximately 30 meters.

The beacons 604 communicate with tag 606 in one embodiment along datalinks illustrated as lines 610 using Ultra-WideBand (UWB) technology asindicated at by UWB ranging signal lines 612. UWB offers the potentialof achieving high ranging accuracy through signal time of arrival (TOA)measurements, even in harsh environments, due to its ability to resolvemultipath signals and to penetrate obstacles. For example, informationrelated to a separation distance between a pair of nodes A and B in aUWB network can be obtained using measurements of signal propagationdelay, or time-of flight (TOF) (1)

$T_{f} = \frac{d}{c}$where d is the actual distance between the two nodes and c is the speedof electromagnetic waves (c 3*10⁸ m/s). Nodes A and B in variousembodiments may be multiple beacons 604, or a beacon 604 and a tag 606,or multiple tags 606. In one embodiment, tags 606 are beacons dedicatedto being mobile position locators.

The IEEE 802.15.4a-2011 standard is the first UWB-based standard forlow-rate wireless networks with localization capability. However otherultrawide band communication techniques may also be employed.

For example, assume a first node A transmits to a second receiving nodeB a packet that contains the timestamp t₁ at which A's packet was sent.Node B receives the packet at time t₂. Under ideal conditions, that is,when node clocks of nodes A and B are perfectly synchronized to a commontime reference, time of flight (T_(f)) can be determined at node B as(2) T_(f)=t₂−t₁. The distance between nodes A and B can be estimated inthis one-way ranging using T_(f). One-way ranging, however, requiresvery accurate synchronization between nodes that is very difficult tomaintain with low cost electronics. For example, a relatively small 10nsec synchronization error between nodes A and B will yield a 3 meterranging error.

In practice, TOF estimation is often done with two-way ranging (TWR)(without a common time reference). In TWR, node transmits a packet tonode B, which replies by transmitting an acknowledgment packet to node Aafter a response delay T_(d). The round trip time (T_(RT)) at node A isdetermined by (3) T_(RT)=2T_(f)+T_(d), which the distance can beestimated assuming T_(d) is known. Knowing T_(d) and T_(RT) allowscalculation of T_(f).

In some embodiments, clock accuracy on both tags 606 and beacons 604 isexpected to be limited in the range of 1 to 10 parts per million (PPM).Clock inaccuracy may generate a significant difference in T_(d) betweena tag 606 and a beacon 604. For example (see equation 3), for a nominalT_(d) value of 0.5 msec, a 10 ppm error yields a 5 nanosecond (nsec)T_(RT) error, which is equivalent to approximately a 0.75 meter rangingerror.

By definition, UWB (IEEE 802.15.4a-2011 based) TWR sequencing is donebetween two nodes. Theoretically, (according to the standard) the numberof measurement that can be done in parallel (on the same channel)without crosstalk is limited to 2 or 4 depending on the channel.

Due to the attenuation and reflections in some environments, such asthose caused by construction materials and the like, it is desirable tohave an unobstructed line of sight from a mobile beacon (such as a tag606) to at least 3 beacons (such as beacons 604) when the mobilebeacon/tag is at any location that should be covered by the system 600.As a result, a large number of beacons 604 may be used.

Use of a large number of beacons 604 may put restrictions on the cost ofinstallation of the beacons 604. It is therefore desirable that thebeacons 604 be at least partially battery powered, and that thecommunication (such as on a low power network 608) from beacons 604 tothe application server 602 be wireless. In some embodiments, beacons mayuse other forms of energy, such as that harvested in known ways, for atleast a part of their power.

A PS such as system 600 may also employ thousands of tags fordetermining the positions of items and/or personnel. For positioningupdates (according to the system 600's or the tag 606's specific updaterate), each of the tags 606 should be involved in at least three TWRsequences with beacons 604 in order to be able to estimate its currentposition. In general, the measurements rate is a multiplication of thenumber of tags 606, the measurements per tag 606, and the update rate.

A TWR round trip time is typically on the order of 1 millisecond. For asystem with thousands of tags 606 and an update rate on the order of fewto tens of seconds, that means that coordination of the TWR measurementis carefully controlled to allow a high number of TWR measurementswithout crosstalk.

In one embodiment, in a TWR, node B is armed, waiting for a poll fromnode A. In order to preserve battery life on node B (either tag orbeacon), it is desirable that node B will start listening just beforesystem A sends the poll. In one embodiment, an application server suchas application server 602 sends scheduling information to the beacons604, such as to node B, so that the beacons 604 are activated forlistening just in time before transfer of a poll. In one embodiment,this is done over the IEEE 802.15.4-2003 compliant network.

It may be desirable that some tags 606 in a system 600 have a lowerupdate rate than others. For example, a tag 606 that is hooked onequipment might operate on a lower update rate than a factory worker,for example if the equipment to which it is attached is less likely tobe mobile than the factory worker, or is likely to move within a certainknown area, or the like. A system 600 in one embodiment monitors aplurality of tags, and updates at least one tag of the plurality of tagsat a rate slower than other tags, depending upon determined conditionsof the at least one tag, such as the equipment with which the at leastone tag is associated.

Another function of the PS 600 is to continuously transfer informationfrom the beacons 604 to a central application station 602. The datatransferred between the beacons 604 and the central application station602 might be either the raw ranging measurements or the calculatedposition of a tag 606 (depending on the configuration of the system600). The information about raw measurements, such as TWR measurementsand the like, is in one embodiment, done over the IEEE 802.15.4-2003compliant network.

In system 600, a system architecture is provided that addresses thechallenges of a positioning system 600 for a high number of tags such astags 606 and beacons such as beacons 604. The system 600 in someembodiments provides one or more advantages. One advantage is efficientcoordination of tag/beacon measurements without cross interference in away that increases the rate of ranging measurements in the system 600.Another advantage is different update rates for different types of tags.Yet another advantage is low power operation, in which an operationsequence for both tags 606 and beacons 604 is designed in a way thatreduces power dissipation, and allows tags 606 and/or beacons 604 tohibernate unless action is to be performed. Another advantage iscalibration of clock drift between beacons 604 and tags 606 in order tocompensate for clock drift between the units' time base.

An architecture for operation on a system such as system 600 is providedin one embodiment as follows:

All beacons 604 operate as nodes on a low power network 608 thatprovides bi-directional communication between the beacons 604 and theapplication server 602. In one embodiment, IEEE 802.15.4-2003 is usedfor this low power network. In one embodiment, the network 608 providesa common time base for all the nodes (i.e., beacons 604) in a way suchthat the entire system 600 is synchronized in a time period of 1 to afew milliseconds, which can be considered standard requirement forcommonly used networks. This timing for synchronization is approximately6 orders of magnitude lower than what is used for UWB ranging.

The radio data link between tags 606 and beacons 604 may be based on UWBradio 612 or on a dedicated data link 610 that operates in anotherstandard technology. Two way ranging (TWR) is in one embodimentinitiated by a tag 606. A tag 606 calculates its position within thearena 620 based on TWR measurements to beacons 604 which are locatednearby, or at least which are in line of sight communication with thetag 606. Once a tag 606 determines its position, the tag reports thatdetermined position to a beacon 604. Positions of tags 606 that arereported to beacons 604 are then transferred to the application server602 via the low power network 608. In one embodiment, each tag 606maintains a list of beacons 604 and the coordinates of each beacon 604.

Each beacon 604 and each tag 606 in the system 600 has, in oneembodiment, a unique system ID. This system ID may be assigned by thecentral application server 602, and allows for the dissemination ofinformation from the central application server 602 to specific beacons604 and/or tags 606, as has been described above.

Each tag 606 has in one embodiment a unique active ID. In one embodimentof the system, the active ID is identical to the system ID. In anotherembodiment, the active ID can be temporal, as part of some registrationprocedure.

A system such as system 600 in one embodiment has a global cycle time onthe order tens of seconds. Every cycle, each of the beacons 604broadcasts a short synchronization message that includes that beacon'sID and coordinates. The synchronization message is sent in a predefinedtime slot in the cycle (in one embodiment a predefined offset time fromthe cycle start time). The predefined time slot for a beacon 604 isassigned in one embodiment by the central application server 602 usingan assignment protocol, such as TDMA as discussed herein.

A beacon 604 message time as an offset from a cycle start can becalculated in one embodiment knowing the beacon's ID. With a knowledgeof the beacon ID, the predefined offset time is known and can becalculated based on a cycle start time.

Tags 606 in one embodiment are continuously active for detection of allor some of the beacon synchronization messages. Upon receipt of asynchronization message by a tag 606, the tag 606 is in completesynchronization with the beacon 604 clock, and is therefore synchronizedto a network global time-base within the network synchronizationaccuracy range.

In one embodiment, a system 600 uses a global registration window havinga length of K time slots. Beacons 604 are active for detection of a tag606's registration message in this window.

In one embodiment, there are unique global time slots assigned in thecycle for each tag 606 active ID. The unique global time slots includeposition report message time slots and range measurement slots. In aposition report message time slot, a tag 606 reports its last (measuredand calculated) position in this time slot. The message reporting thelast tag position may be received by one or more beacons 604. In a rangemeasurement slot, a tag 606 may perform range measurement to beacons 604in its vicinity, or that are within a line of sight of the tag 606. Therange measurement slot is a global time slot, in one embodiment in alength of n TWR measurements per tag 606. Following tag 606 registrationin the global registration window, beacons 604 are active for detectionof tag TWR measurements in this slot. The TWR measurements in oneembodiment occur as polls including the range measurements.

A 30 second cycle time example 700 is shown in graphical form in FIG. 5.Sequences of operation for the various time slots and operations withina cycle 700 are shown. Cycle 700 begins at time 702, and ends at time704. At time 702, frame start 706 occurs for tag registration. In tagregistration, tags 606 are continuously listening at 708 forsynchronization messages from beacons 604. In one embodiment, 200 timeslots are available for beacon 604 time synchronization. After receivinga beacon 604's synchronization message for the first time, a tag 606sends a registration message at 710. The message is sent randomly in oneof the allotted number of time slots (in one embodiment 100 slots). Uponreceiving a registration message, the beacon 604 sends a confirmationmessage to the tag 606 at 712. In one embodiment, the beaconacknowledgement to tag registration occurs in 200 time slots. Thebeacons 604 begin listening on the tag position reporting slots at 714(in one embodiment the tag position reporting time slots are arranged inblocks of 1000 and are sequential).

Range measurements are performed as follows. In range measurement, abeacon 604 is always listening on a registered tag measurements slot 714for a registered tag poll. A tag 606 may choose to poll for TWR to aspecific beacon 604 during this time. That tag 606 can poll to up to nbeacons 604 in a single cycle 700.

Beacon 604/tag 606 clock drift compensation is performed in oneembodiment to reduce the response delay (T_(d)) error. Both beacon 604and tag 606 have a systematic clock drift against a network clock. Inone embodiment, a sequence for calibrating and compensating for thisdrift is as follows, using the example shown in FIG. 6, and determiningT_(d) between a portable unit PU (such as a tag 606) and beacon B (suchas a beacon 604). Beacon B sends its internal clock count as part of asynchronization message 708 (B_(c) ^(n)) in a cycle n within a globalwindow 800. The PU (e.g., tag 606) logs its internal clock count uponthe arrival of the synchronization message (T_(c) ^(n)) in cycle n. Thetag 606 calculates a correction factor using equation (4)

$D = {\frac{T_{c}^{n} - T_{c}^{n - 1}}{B_{c}^{n} - B_{c}^{n - 1}}.}$This correction factor is used by the tag 606 to correct the time delayT_(d using equation) (5) T_(RT)=2T_(f)+T_(d)*D which replaces equation(3). The corrected time delay compensates for the clock drift, allowingfor improved position location for tags 606 in the system 600.

Although the present disclosure has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the disclosure. Embodiments of the present disclosure canbe used in indoor, outdoor or hybrid environments. Communication can bein accordance with any standard and is not limited to a mesh networktype communication system. The term “RF” refers to radio frequency ofany appropriate wavelength. As used herein, the term “anchor” refers toa base transmitter whose location is known and is used as a referencelocation in determining location. The term “mobile device” refers to thedevice, such as a mobile station, whose location is being identified.The processing unit which is used to determine location may reside inthe mobile station, in one or more of the anchor stations, at a centralstation, or at some other location.

An optional GPS module such as module 504 shown above, may be providedin a mobile station (such as a tag 606) and may be used when a GPSsignal is available. A low power communication protocol (i.e., 608) suchas those based on the IEEE 802.15.4-2003 physical layer may be used as abackbone for a positioning system that uses robust ranging achievedthrough the use of a low power UWB ranging and communications protocolsuch as those based on IEEE 802.15.4a-2011. However, other ranging andcommunication protocols and techniques may be used to implement theembodiments of the disclosure. The configuration provides accurate lowpower location detection that is substantially immune from multipatherrors. Novel arbitration techniques allow location monitoring of manythousands of tags. The backhaul communication between beacons and theserver preferably employs a low power communication technique such asWirelessHART®, ISA100, Zigbee® and Bluetooth® Low Energy, and LORA®based WAN, or others.

What is claimed is:
 1. A two-way ranging based positioning system,comprising: a plurality of active tags each having a position; aplurality of fixed location beacons configured for location of aposition of a tag of the plurality of active tags using two-way rangingon a first network; and a processing unit coupled via a data networkseparate from the first network to the plurality of beacons, the beaconsconfigured to receive tag positions from the plurality of active tags onthe first network, and the processing unit configured to receiveposition information of the plurality of active tags from the pluralityof beacons on the data network and to determine and transfer rangingtime slots to the plurality of beacons on the data network for two wayranging; and an application server configured to be operated by theprocessing unit; wherein the plurality of active tags and the pluralityof beacons are synchronized continuously to a common time base forranging during assigned time slots; and wherein the beacons and the tagsperform two-way ranging therebetween in the assigned time slots by:ranging the active tag to at least a portion of the plurality of beaconsduring the beacon assigned ranging time slots; and reporting a positionof the active tag to at least a beacon of the plurality of beacons; andwherein the application server configured to determine a position of anactive tag by: providing the common time base for the plurality of fixedlocation beacons; receiving a synchronization message from each of theplurality of fixed location beacons in a fixed time slot for eachbeacon; registering the active tag to the common time base based on thereceived synchronization message; and receiving active tag ranginginformation from at least one of the plurality of beacons driftcompensate the plurality of beacons and the active tag, using aninternal clock count B_(c) ^(n) of a beacon synchronization message andan internal clock count T_(c) ^(n) of the active tag for a currentupdate cycle n of the system, by determining a correction factor D tocorrect for clock drift versus the common time base using the formula${D = \frac{T_{c}^{n} - T_{c}^{n - 1}}{B_{c}^{n} - B_{c}^{n - {1 \cdot}}}};$and wherein time of flight between a beacon and the active tag is T_(f),wherein a response delay between a beacon and an active tag is T_(d),and wherein the application server is further configured to apply thecorrection factor to a two-way ranging round trip flight time asT_(RT)=2T_(f)+T_(d)*D.
 2. The positioning system of claim 1, whereineach beacon of the plurality of beacons listens for a two-way rangingpoll from a tag of the plurality of active tags in a predefined window.3. The positioning system of claim 1, wherein the plurality of activetags has a global reporting time slot.
 4. The positioning system ofclaim 1, wherein each tag of the plurality of active tags is configuredto determine its position based on two-way ranging between it and atleast one beacon of the plurality of beacons.
 5. The positioning systemof claim 4, wherein each tag of the plurality of tags is configured todetermine its position based on two-way ranging between it and at leastthree beacons.
 6. The positioning system of claim 1, wherein theprocessing unit is further configured to update positions of theplurality of tags at a rate dependent upon determined conditions of thetags.
 7. The two-way ranging based positioning system of claim 1,wherein the application server is further configured to register theactive tag by: listening by the active tag for synchronization messagesfrom the plurality of beacons; sending a registration message uponreceipt of a synchronization message from a beacon of the plurality ofbeacons; receiving a confirmation message from the beacon of theplurality of beacons at the tag; and initiating listening for a tagposition report in a tag position report time slot.
 8. The two-wayranging based positioning system of claim 1, wherein the applicationserver is further configured to provide a synchronization message byproviding the synchronization message within a range of predeterminedbeacon time synchronization time slots in an update cycle for thesystem.
 9. The two-way ranging based positioning system of claim 1,wherein the application server is further configured to register theactive tag by registering the active tag within a range of predeterminedtag registration time slots in an update cycle for the system.
 10. Thetwo-way ranging based positioning system of claim 9, wherein theapplication server is further configured to range the active tag byranging the active tag within a range of time slots in the update cycleafter the tag registration time slots.
 11. The two-way ranging basedpositioning system of claim 1, wherein the application server is furtherconfigured to range the active tag by: issuing from the active tag atwo-way ranging signal; receiving at the active tag return signals fromat least three beacons of the plurality of beacons; and calculating aposition of the active tag at the active tag using the common time baseand the received return signals.
 12. The two-way ranging basedpositioning system of claim 11, wherein the application server isfurther configured to issue a two-way ranging signal by issuing to adetermined one of the plurality of beacons.
 13. The two-way rangingbased positioning system of claim 12, wherein the application server isfurther configured to range to more than one beacon of the plurality ofbeacons within an update cycle of the system.
 14. The two-way rangingbased positioning system of claim 11, wherein the application server isfurther configured to issue two-way ranging signals within a globallydesignated window in an update cycle of the system.
 15. The two-wayranging based positioning system of claim 1, and the application serveris further configured to: connect the plurality of beacons and theapplication server with a wireless communication network; and transfermobile tag information from the plurality of beacons to the applicationserver.
 16. The two-way ranging based positioning system of claim 1,wherein the system monitors a plurality of tags, and the applicationserver is further configured to update at least one tag of the pluralityof tags at a rate slower than other tags, depending upon determinedconditions of the at least one tag.
 17. The system of claim 1, whereinthe first network comprises an ultrawide band network.
 18. A positioningsystem, comprising: a plurality of active tags; a plurality of fixedposition beacons in communication with the plurality of active tags overa two-way ranging based network; wherein each active tag of theplurality of active tags in the system has a globally designated windowfor two-way ranging polls between it and at least one of the pluralityof beacons, wherein the plurality of beacons accepts two-way rangingpolls within the globally designated window for each active tag, andwherein the plurality of active tags each report positions within asingle global reporting time slot over the two-way ranging basednetwork; a processing unit coupled via a data network separate from thetwo-way ranging based network to the plurality of beacons, theprocessing unit configured to receive position information of theplurality of active tags from the plurality of beacons on the datanetwork and to determine and transfer ranging time slots to theplurality of beacons on the data network for two-way ranging; and anapplication server configured to be operated by the processing unit;wherein the beacons and the tags perform two-way ranging therebetween inthe assigned time slots by: ranging the active tag to at least a portionof the plurality of beacons during the beacon assigned ranging timeslots; and reporting a position of the active tag to at least a beaconof the plurality of beacons; and wherein the application serverconfigured to determine a position of an active tag by: providing thecommon time base for the plurality of fixed location beacons; receivinga synchronization message from each of the plurality of fixed locationbeacons in a fixed time slot for each beacon; registering the active tagto the common time base based on the received synchronization message;and receiving active tag ranging information from at least one of theplurality of beacons drift compensate the plurality of beacons and theactive tag, using an internal clock count B_(c) ^(n) of a beaconsynchronization message and an internal clock count T_(c) ^(n) of theactive tag for a current update cycle n of the system, by determining acorrection factor D to correct for clock drift versus the common timebase using the formula${D = \frac{T_{c}^{n} - T_{c}^{n - 1}}{B_{c}^{n} - B_{c}^{n - {1 \cdot}}}};$and wherein time of flight between a beacon and the active tag is T_(f),wherein a response delay between a beacon and an active tag is T_(d),and wherein the application server is further configured to apply thecorrection factor to a two-way ranging round trip flight time asT_(RT)=2T_(f)+T_(d)*D.
 19. The system of claim 18, wherein the two-wayranging based network comprises an ultrawide band network.